The death toll has grown to fourteen from Saturday's massive landslide near Oso, Washington, located about 50 miles north-northeast of Seattle. At least seven were injured, and 176 are listed as missing, though this total is likely to decrease dramatically as missing people check in. The landslide was triggered by unusually heavy rains over the past 30 days in the region. A personal weather station located about ten miles west of the slide recorded 13.81" of precipitation in the 30 days prior to the slide, including 5.17" in the ten days just before. Precipitation imagery from NOAA's Advanced Hydrological Precipitation Service (Figure 2) shows that the 30-day precipitation amounts in the region were more than 8" above average--about double the usual amount of rain for this time of year.
Figure 1. The Oso, Washington area before the March 22, 2014 landslide as seen on Google Earth (top) and after the landslide, as photographed by the Washington Department of Transportation (bottom.) The landslide blocked the North Fork of the Stillaguamish River and Highway 530.
Figure 2. Precipitation for the 30-day period ending March 24, 2014, was 150% - 200% of average in Oso, Washington--about 8" above average. Image credit: NOAA/AHPS.
Figure 3. Predicted precipitation for the 7-day period ending on Tuesday, April 1, 2014. The landslide area to the northwest of Seattle is expected to receive 2 - 4" of precipitation, which will slow recovery efforts from the landslide. However, the rains over Northern California will be welcome, helping to fill drought-depleted reservoirs as that state's dry season approaches. Image credit: NOAA/HPC.
A re-activation of an old landslide
According to Dave Petley, Professor of Hazard and Risk in the Department of Geography at Durham University in the United Kingdom, it is clear that major landslides have occurred here on many previous occasions, so much so that the landslide is known as either the Hazel landslide or the Steelhead landslide. In his excellent Landslide Blog, my go-to source of information for any landslide, he writes: "The landslide has been widely reported as a mudslide. In terms of the lower portion, which did the damage, this is correct, although in places it might have been more of a mudflow than a mudslide. However, the upper portion is a rotational landslide–the rotated block with the fallen trees is very clear. A working hypothesis would be that this block failed catastrophically, transferring load onto the block below, which in turn generated very high pore water pressures, causing fluidisation and a very rapid mudflow that struck the settlements across the river." He writes that the last event on a similar scale he knows of was the 25th December 2003 debris flow in San Bernadino County, California, which killed sixteen people. Weather historian Christopher C. Burt has a post about the worst landslides in U.S. history, which puts this week's landslide in context.
The Yakima Herald has a very nice article that details the chronology of events on the Oso landslide. This includes:
• 1949: A large landslide (1000 feet long and 2600 feet wide) affected the river bank
• 1951: Another large failure of the slope; the river was partially blocked
• 1967: Seattle Times published an article that referred to this site as “Slide Hill”
• 1997 report, by Daniel Miller, for the Washington Department of Ecology and the Tualialip Tribes
• 1999: US Army Corps of Engineers report by Daniel and Lynne Rodgers Miller that warned of “the potential for a large catastrophic failure”
• 25 January 2006: large movement of the Steelhead landslide blocked the river
When Will Spring Come For REAL? Join Me at 5:30pm EDT Tuesday for a Google Hangout Discussion
Spring has officially begun for those of us in the Northern Hemisphere, but it's still more like winter in the Midwest and Northeast U.S. In celebration of the new season, I'll host a 15-minute Google Hangout on Tuesday, March 25th at 5:30pm EDT to review where on Earth the most severe winter weather is occurring, and forecast when those of us still experiencing winter can expect to see Spring--for real! I'll focus on the forecast for four cities: Detroit, Boston, Seattle, and Moscow, and discuss some of the remarkable weather events those cities have seen this month. You can watch the hangout by visiting our Weather Underground Spring Forecast page.